Checkbox addition

Hi, I've created a simple list of questions with checkboxes next to them. I have created a 'total' box at the bottom to provide the number of checkboxes selected. I've successfully got it to add up using sum+ and with a different approach that I test using JavaScript. It all works fine in Adobe. The problem is when the PDF is opened in a browser or other PDF reader. The checkboxes can be selected but the count stays at zero. I've not idea why it would work in Adobe but nowhere else. I'm totally stuck. Any help gratefully received.